Field of the Invention
The present invention relates to a pneumatic tire in which shallow grooves are formed on a surface of a land portion of a tread surface.
Description of the Related Art
In a winter tire or an all-season tire, a rubber having a high friction coefficient is employed in a tread surface for the purpose of enhancing a traveling performance on an ice road surface. However, in order to sufficiently achieve an effect thereof, a running-in travel for about several hundreds of km is required. Accordingly, in order to achieve the performance from an initial state in which the running-in travel is not performed, there has been known an application of a so-called first edge process which forms shallow grooves on a surface of a land portion of the tread surface. Even in a summer tire, there is a case that the first edge process is applied for the purpose of improving the traveling performance on a wet road surface.
As the first edge process, there have been known a mode that the shallow grooves are linearly formed as disclosed in patent document 1, and a mode that the shallow grooves are formed like a grid as disclosed in patent document 2. However, since the formation of the shallow grooves lowers a rigidity of the surface portion of the land portion, the surface portion of the land portion tends to locally topple at the braking time or the turning time. As a result, a ground contact performance is deteriorated, and there is a case that an edge effect caused by the shallow grooves can not be sufficiently achieved. Therefore, the performance can be expected to be improved by suppressing the toppling of the surface portion of the land portion.
Patent document 3 describes a structure in which oval cuts are concentrically formed on the tread surface. However, it is difficult to densely arrange a plurality of cuts each having the oval shape. Furthermore, patent document 4 describes a structure in which polygonal shallow grooves are concentrically formed as the first edge process. However, since the structure is only formed independently of each of blocks, a density at which the shallow grooves are formed is lower than that in the modes in the patent documents 1 and 2, and there has been room for improvement on the achievement of the edge effect by the shallow grooves.